How often should self-monitoring of blood pressure be repeated? A secondary analysis of data from two randomized controlled trials.

Background: Little evidence exists around the optimal frequency of self-monitoring of blood pressure (BP). Testing too frequently can lead to erroneous management changes due to random "noise" leading to raised measurements by chance. This study used recent trial data to evaluate self-monitored BP over time, aiming to determine how frequently patients should self-monitor.

Methods: Data from patients with home BP ≤135/85 mmHg on stable medication in the self-monitoring groups of two trials were analysed using a mixed effects model. The primary outcome was mean change in BP per month. Secondary outcomes included intra-individual BP variability, and probability of truly raised BP over time.

Results: 232 participants from HOMEBP, and 582 participants from TASMINH4 were included. The mean changes in systolic BP per month per study were -0.1 mmHg [standard deviation (SD) 0.6 mmHg], and -0.2mmHg [SD 0.7 mmHg], respectively. Intra-individual systolic variability (SD) per month was 4.7 and 5.1 mmHg respectively. Using TASMINH4 data, from a starting systolic BP of 130 mmHg, re-testing BP after 6 months resulted in a probability of 18% that BP ≥135 mmHg, with a 25% probability that this reflected truly raised BP; after 12 months the probability of a raised reading was 26% with a 65% probability this reflected a true rise.

Conclusions: In the absence of medication changes, there was very little change in mean self-monitored BP per month, with larger variability within an individual's monthly submitted readings. For people with initially controlled BP and stable medication, repeating self-monitoring at 12 months is likely to be appropriate in guiding management.